Fatty acid synthesis and associated lipogenic enzymes of rat liver are suppressed far more effectively by polyunsaturated fats (PUFA) than by saturated fats (SFA). This effect is independent of type or amount of carbohydrate consumed. Phosphofructokinase and pyruvate kinase activities are not influenced by PUFA, which indicates PUFA action is specific for lipogenesis. The reduced fatty acid synthesis is accompanied by a slower rate of malonyl-CoA formation and these changes occur before a reduction in maximal velocity of acetyl-CoA carboxylase or fatty acid synthetase. However, within four days the activity of carboxylase has dropped to a level which is nearly identical to rate of fatty acid synthesis. The mechanism of PUFA apparently involves both an acute negative feedback on substrate flux to fatty acids and a long-term effect on the level of lipogenic enzymes. In contrast to the rat, PUFA and SFA inhibit chick liver lipogenesis with similar efficacy.